Returns the first parameter if not undefined, otherwise the second parameter.
Useful for setting a default value for a parameter.
    
    
    
    
    
    
        
| Name | Type | Description | 
|---|---|---|
| a | * | |
| b | * | 
Returns:
    Returns the first parameter if not undefined, otherwise the second parameter.
        
    
    
Example:
param = Cesium.defaultValue(param, 'default');